Propylene polymer compositions from common propylene polymers and β-nucleating agents are known. β-Nucleating agents are useful in compositions for forming unstretched films which can be made porous by extraction of β-spherulites and stretching the films (disclosed for example in U.S. Pat. No. 4,386,129; U.S. Pat. No. 4,975,469).
Another effect of β-nucleating agents in propylene compositions is the improved thermoformability. As the melting point of the β-form of the spherulites of polypropylene based resins is generally about 144 to 148° C., contrasted with the typical melting point range of α-form spherulites of about 159 to 163° C., melt forming from these compositions is possible at lower temperatures and higher production rates (WO 93/12 262).
The disadvantage of these compositions having good impact properties is that they show unsatisfactory strength.
Conventional polypropylene homopolymers usually show high stiffness, but poor impact strength. Conventional β-nucleated polypropylenes usually have improved impact strength but also exhibit reduced stiffness.